1. Field of the Invention
The present invention relates to a method for estimating transmission channel characteristics using a pilot signal, and more particularly, to an estimation method suitable for a multicarrier modulation scheme.
2. Description of the Background Art
In wireless or wired communications, multicarrier modulation is widely used. The multicarrier modulation scheme is a method for dividing and assigning transmission data to a plurality of carriers and modulating and multiplexing the carriers with the respective assigned pieces of the transmission data. OFDM (Orthogonal Frequency Division Multiplexing), which is a type of multicarrier modulation scheme, is widely in practical use in fields, such as digital terrestrial television broadcasting, wireless LAN (Local Area Network), xDSL (x Digital Subscriber Line), PLC (Power Line Communications), and the like. Examples of specific applications include DVB-T for digital terrestrial television broadcasting, IEEE802.11a for wireless LAN, ADSL for xDSL, HomePlug for powerline communications, and the like. This OFDM is referred to as an OFDM/QAM (Quadrature Amplitude Modulation) multicarrier modulation scheme or simply OFDM/QAM when it is distinguished from other multicarrier modulation schemes.
(OFDM/QAM)
The principle of OFDM/QAM is described in S. B. Weinstein and Paul M. Ebert, “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform”, IEEE Transaction on Communications, vol. COM-19, pp. 628-634, October 1971.
OFDM/QAM is a multicarrier modulation scheme of subjecting a plurality of carriers to complex vector modulation in units of each symbol for multiplexing. A symbol interval includes a guard interval and an effective symbol interval. In wireless mobile communications, multipath propagation often disadvantageously occurs, i.e., a plurality of incoming transmission signals which are variously delayed due to reflection of radio waves are received. Since OFDM/QAM has the guard interval, differences in arrival time of a plurality of incoming waves due to multipath propagation are tolerated and orthogonality is maintained between a plurality of carriers, so that a transmission signal can be received without occurrence of inter-symbol interference or inter-carrier interference. In the field of wireless communications, such as digital terrestrial television broadcasting, wireless LAN and the like, such resistance of OFDM/QAM to multipath propagation is utilized.
However, in OFDM/QAM, the guard interval is only used for toleration of multipath propagation, but no effective information is transmitted during the guard interval. Therefore, OFDM/QAM disadvantageously has low spectral efficiency or otherwise loss. Also in OFDM/QAM, when a delay time difference due to multipath propagation exceeds the guard interval, transmission quality is rapidly deteriorated, resulting in an insufficient effect of filtering a narrow-band interference signal.
As another multicarrier modulation scheme for solving the above-described problem, an OFDM/OQAM (Offset Quadrature Amplitude Modulation) multicarrier modulation scheme is known. The OFDM/OQAM multicarrier modulation scheme is hereinafter simply referred to as OFDM/OQAM.
(OFDM/OQAM)
The principle of OFDM/OQAM is described in Burton R. Saltzberg, “Performance of an Efficient Parallel Data Transmission System”, IEEE Transaction on Communications, vol. COM-15, pp. 805-811, December 1967.
OFDM/OQAM is a multicarrier modulation scheme of subjecting a plurality of carriers to amplitude modulation in units of each symbol for multiplexing. In OFDM/OQAM, a difference in modulation phase between adjacent symbols or adjacent carriers is set to be π/2 radians, and a symbol time interval Ts and a carrier frequency interval fs have a relationship represented by Ts=1/(2fs). Note that, thus, in OFDM/OQAM, amplitude modulation is applied to phase axes which alternatively become orthogonal in symbol time intervals each of which is about half of that of OFDM/QAM when the carrier frequency interval fs is the same.
Modulation schemes which have features of OFDM/OQAM and to which the present invention is applicable are all handled as OFDM/OQAM. For example, time-limited orthogonal multicarrier modulation (see R. Li and G. Stette, “Time-Limited Orthogonal Multicarrier Modulation Schemes”, IEEE Transactions on Communications, vol. 43, pp. 1269-1272, February/March/April 1995), DWMT (Discrete Wavelet Multitone, see M. A. Tzannes, M. C. Tzannes, J. Proakis and P. N. Heller, “DMT Systems, DWMT Systems and Digital Filter Banks”, IEEE International Conference on Communications, pp. 311-315, May 1994), extended lapped transform (see H. S. Malvar, “Extended Lapped Transforms: Properties, Applications, and Fast Algorithms”, IEEE Transactions on Signal Processing, Vol. 40, Noll, pp. 2703-2714, November 1992), and a multicarrier transmission method (DWMC transmission method) which performs a digital modulation/demodulation process using a real coefficient wavelet filter bank (see Japanese Unexamined Patent Application Publication Nos. 2005-303960 (Patent Document 1) and 2005-311413 (Patent Document 2)), are regarded as being of the same type and are collectively referred to as OFDM/OQAM.
FIG. 14 is a diagram illustrating exemplary time and frequency responses of OFDM/OQAM multicarrier modulation signals. A portion (a) of FIG. 14 illustrates the time responses of the OFDM/OQAM multicarrier modulation signals, and a portion (b) of FIG. 14 illustrates the frequency responses of the OFDM/OQAM multicarrier modulation signals.
In the portion (a) of FIG. 14, the horizontal axis represents times and the vertical axis represents amplitudes. Time responses St−1, St and St+1 of three symbols of OFDM/OQAM multicarrier modulation signals are separately indicated. The duration of the time response of one symbol of OFDM/OQAM multicarrier modulation signal extends over a plurality of symbol time intervals (four Ts's in the example of the portion (a) of FIG. 14), so that the time responses of the OFDM/OQAM multicarrier modulation signals overlap each other in the time-axis direction. The time response duration of the OFDM/OQAM multicarrier modulation signals is referred to as an overlapping symbol number. Therefore, in the example of the portion (a) of FIG. 14, the overlapping symbol number is four. Here, a symbol sampling length is KM where K represents the overlapping symbol number and M represents the total number of subcarriers.
In the portion (b) of FIG. 14, the horizontal axis represents frequencies and the vertical axis represents spectral intensities. Subcarrier modulation spectrum fc1 to fc3 which are frequency responses when three subcarriers are modulated with respective pieces of transmission data are separately indicated. As can be seen from the portion (b) of FIG. 14, the subcarrier modulation spectrum fc1 to fc3 overlap each other. Thus, since the OFDM/OQAM multicarrier modulation signals overlap both in the time and frequency directions, an OFDM/OQAM modulation signal of each symbol is interfered with by OFDM/OQAM modulation signals of its preceding and succeeding symbols and its adjacent subcarriers. However, in OFDM/OQAM, since only an imaginary-axis component which is orthogonal to a real-axis component (amplitude component) which is modulation information is interfered with by an OFDM/OQAM modulation signal, OFDM/OQAM multicarrier modulation signals maintain orthogonality with each other.
On the other hand, in systems to which OFDM/QAM is applied, a frame format is used in which, in order to estimate transmission channel characteristics and frequency and phase errors between a transmitter and a receiver, and the like, a reference signal called a pilot signal which is known both in the transmitter and the receiver is inserted. An example of such a system to which OFDM/QAM is applied is a wireless LAN standard IEEE802.11a.
In OFDM/QAM, orthogonality is maintained so that there is no interference both between each symbol and between each subcarrier of OFDM/QAM modulation signals. Therefore, as illustrated in FIG. 15A, in a time-frequency plane in which OFDM/QAM modulation signals are arranged, a modulation signal can be used in which pilot signals are freely provided in units of one symbol and one subcarrier. FIG. 15B is a diagram illustrating a modulation vector and a demodulation vector of a pilot signal on a complex plane when it is assumed that no variation or noise is present in a transmission channel in OFDM/QAM. In this case, the modulation vector and the demodulation vector are equal to each other. Thus, in OFDM/QAM, since the modulation vector and the demodulation vector of a pilot signal are equal to each other under the environment that there is no variation in a transmission channel, the receiver can predict the demodulation vector of the pilot signal. A known demodulation vector in the receiver is hereinafter referred to as an ideal demodulation vector. In OFDM/QAM, by calculating the amount of a change between the ideal demodulation vector and a demodulation vector of an actually received pilot signal, transmission channel characteristics can be estimated.
However, in OFDM/OQAM, a modulation signal in which pilot signals are freely arranged in units of one symbol and one subcarrier cannot be used, as is similar to OFDM/QAM. As described above, in OFDM/OQAM, each OFDM/OQAM multicarrier modulation signal interferes with imaginary-axis components of OFDM/OQAM modulation signals of its preceding and succeeding symbols and its adjacent subcarriers. Therefore, even if a subcarrier assigned as a pilot carrier is modulated with a pilot signal having a known amplitude which includes only a real-axis component, the subcarrier is interfered with OFDM/OQAM multicarrier modulation signals of its preceding and succeeding symbol and its adjacent subcarriers, so that an imaginary-axis component occurs in the demodulation vector of the pilot signal. This will be described with reference to FIG. 16.
FIG. 16 is a diagram illustrating a range on a complex plane within which an ideal demodulation vector is present when a pilot signal including only a real-axis component having an amplitude value of “1” is received and demodulated under the environment that no variation or noise is present in a transmission channel in OFDM/OQAM. As illustrated in FIG. 16, an ideal demodulation vector of a pilot signal including only a real-axis component having an amplitude value of “1” is present anywhere on a straight line L0 of FIG. 16 and is not uniquely fixed. This is because, as described above, there is an interference from an imaginary-axis component received from OFDM/OQAM multicarrier modulation signals of preceding and succeeding symbols and adjacent subcarriers, and the amount of the interference occurring in the imaginary-axis component depends on values possessed by the OFDM/OQAM multicarrier modulation signals of the preceding and succeeding symbols and the adjacent subcarriers.
Thus, in OFDM/OQAM, even if a pilot signal having a known amplitude is transmitted as in OFDM/QAM, the ideal demodulation vector of the pilot signal has an uncertain imaginary-axis component caused by an interference from an OFDM/OQAM multicarrier modulation signal which is not known to a receiver for preceding and succeeding symbols and adjacent subcarriers, and therefore, is not uniquely fixed. Therefore, the receiver cannot obtain the amount of a change between the ideal demodulation vector of the pilot signal and a demodulation vector of an actually received pilot signal, as is similar to OFDM/QAM, so that transmission channel characteristics cannot be correctly estimated.
Methods for estimating transmission channel characteristics in OFDM/OQAM, which solves the problem, have been proposed in, for example, Patent Documents 1 and 2.
In conventional multicarrier modulation disclosed in Patent Document 1, three methods for estimating transmission channel characteristics are described. In a first method of Patent Document 1, a continuous pilot signal is used which is obtained by modulating all subcarriers over 2K−1 symbols (K represents an overlapping symbol number) with the same pilot signal.
Initially, the first method of Patent Document 1 will be described.
FIGS. 17A and 17B are conceptual diagrams illustrating a pilot signal in the first method of Patent Document 1. FIG. 17A illustrates a portion of a frame format used in the first method. FIG. 17B is a schematic diagram illustrating a relationship in frequency between spectrum of pilot signals and subcarriers. In FIG. 17A, the horizontal axis represents an arrangement of subcarriers in a frequency direction, and the vertical axis represents an arrangement of symbols in a time direction. As indicated by double hatching in FIG. 17A, in the first method, a continuous pilot signal is obtained by modulating all subcarriers over 2K−1 symbols with the same pilot signal.
In FIG. 17B, the horizontal axis represents frequencies and the vertical axis represents spectral intensities. Spectrum of the continuous pilot signal appear as emission line spectrum between subcarrier modulation spectrum. FIG. 17B illustrates, for example, only spectrum fp1 to fp3 of the continuous pilot signal. The reason why the frequency relationship between the spectrum of the continuous pilot signal and the subcarrier modulation spectrum is what is illustrated in FIG. 17B, is that, since the continuous pilot signal is composed of the same pilot signal which is repeated in the time intervals Ts, the spectrum of the continuous pilot signal repeatedly appears at intervals of 1/Ts on the frequency axis, while the frequency interval of the subcarrier modulation spectrum is 1/(2Ts).
Also, since the overlapping symbol number is K, the K-th symbol and thereafter of the continuous pilot signal constitute a section in which there is no overlap with an unknown OFDM/OQAM multicarrier modulation signal in preceding and succeeding receivers. Therefore, in the ideal demodulation vectors of the pilot signals included in the continuous pilot signal, no interference occurs in the uncertain imaginary-axis direction due to the unknown OFDM/OQAM multicarrier modulation signal in the receiver, so that the ideal demodulation vector of the pilot signal is fixed to a single known point in the receiver. Therefore, in the first method of Patent Document 1, by calculating the amount of a change between a known ideal demodulation vector and a demodulation vector of an actually received pilot signal in the receiver, transmission channel characteristics are estimated.
FIG. 18 is a conceptual diagram illustrating a method for estimating transmission channel characteristics in the first method of Patent Document 1. In FIG. 18, an ideal demodulation vector (a point A) of a pilot signal and a demodulation vector (a point B) of an actually received pilot signal are illustrated on a complex plane. Since the ideal demodulation vector of the pilot signal is fixed to the single point A, the amount of a change between the ideal demodulation vector and the demodulation vector of the actually received pilot signal can be calculated.
Thus, by using a continuous pilot signal which is obtained by modulating all subcarriers over 2K−1 symbols with the same pilot signal, the ideal demodulation vector of a pilot signal is fixed to a single point, so that transmission channel characteristics can be estimated.
Next, a second method of Patent Document 1 will be described.
FIG. 19 is a diagram illustrating a portion of a frame format used in the second method. In FIG. 19, the horizontal axis represents an arrangement of subcarriers in the frequency direction, and the vertical axis represents an arrangement of symbols in the time direction. In the second method, a short continuous pilot signal is used which is obtained by modulating all subcarriers over one or more and less than 2K−1 symbols (three symbols in FIG. 19) with a pilot signal.
The short continuous pilot signal having a time corresponding to one or more and less than 2K−1 symbols overlaps an unknown OFDM/OQAM multicarrier modulation signal, which presents in preceding and succeeding of the short continuous pilot signal, in a receiver in any section. Therefore, the ideal demodulation vector of the pilot signal is invariably interfered with by an uncertain imaginary-axis component due to preceding and succeeding OFDM/OQAM multicarrier modulation signals. Therefore, the ideal demodulation vector of the pilot signal is not fixed to a single point, and therefore, is unknown to a receiver. Therefore, in the second method, by utilizing the fact that two adjacent subcarriers have substantially the same transmission channel characteristics, transmission channel characteristics are estimated based on the ideal demodulation vectors of pilot signals of two adjacent subcarriers at the same time.
FIG. 20 is a conceptual diagram illustrating a method for estimating transmission channel characteristics in the second method of Patent Document 1. In FIG. 20, a signal point which is obtained from a demodulation vector in an m-th subcarrier (any one subcarrier in FIG. 19) is represented by a point Rm, and a signal point which is obtained from a demodulation vector in an(m+1)-th subcarrier adjacent to the m-th subcarrier is represented by a point Rm+1. A straight line L0 in FIG. 20 represents a line connecting two signal points which are obtained from the ideal demodulation vectors of pilot signals obtained by demodulating pilot carriers of the m-th subcarrier and the (m+1)-th subcarrier when there is no variation or noise in a transmission channel. As illustrated in FIG. 20, a straight line extending through the point Rm and the point Rm+1 is indicated by L1, and an intersection point of a perpendicular line dropped from the origin O to the straight line L1 and the straight line L1 is indicated by P. By obtaining a distance between the intersection point P and the origin O and an angle θ between the I axis and the line segment OP to calculate the amounts of changes in amplitude and phase, transmission channel characteristics are estimated.
Also, since an interference component in the imaginary-axis direction occurring in a demodulation vector of a pilot signal depends on an unknown OFDM/OQAM multicarrier modulation signal in a receiver for an adjacent subcarrier, a distance between demodulation vectors of two pilot signals may be small. When the distance between demodulation vectors of two pilot signals is small, a demodulation error between the points Rm and Rm+1 has a significant influence on the slope of the straight line L1, so that, particularly, the precision of estimation of the amount of a change in phase is deteriorated. Therefore, in the second method, the numbers of repetitions of a pilot symbol is increased, thereby increasing the distance between demodulation vectors of two pilot signals. As the numbers of repetitions of a pilot symbol is increased, the influence of overlapping of an unknown OFDM/OQAM multicarrier modulation signal in a receiver becomes smaller. Thereby, as is similar to the spectrum of a pilot signal in the first method, the spectrum of a short continuous pilot signal of one or more and less than 2K−1 symbols is close to an emission line spectrum having a component at an intermediate frequency between two subcarrier modulation spectrum. Therefore, an interference from an OFDM/OQAM multicarrier modulation signal of an adjacent subcarrier is reduced. Therefore, the influence of the demodulation error between the point Rm and the point Rm+1 on the slope of the straight line L1 is reduced, resulting in an improvement in the precision of estimation of transmission channel characteristics.
Thus, a transmission channel can be estimated based on demodulation vectors of pilot signals of two adjacent subcarriers at the same time. Also, by increasing the numbers of repetitions of a pilot signal to increase the distance between demodulation vectors of two pilot signals, the precision of estimation of a transmission channel is improved.
Next, a third method of Patent Document 1 will be described.
In the third method, a short continuous pilot signal is used which is obtained by modulating all subcarriers over two or more and less than 2K−1 symbols with a pilot signal. In addition, by utilizing the fact that demodulation vectors of two successive pilot signals in the time direction in the same subcarrier suffer from substantially the same transmission channel distortion, transmission channel characteristics are estimated based on the demodulation vectors of two successive pilot signals in the time direction in the same subcarrier. In this case, although the number of repetitions of a pilot signal needs to be two or more, transmission channel characteristics can be estimated based on pilot signals of two successive symbols in the same subcarrier.
Thus, a transmission channel can be estimated based on demodulation vectors of pilot signals of two successive symbols in the same subcarrier.
Next, Patent Document 2 will be described.
In Patent Document 2, two adjacent subcarriers are successively modulated with the same pilot signal.
FIGS. 21A and 21B are conceptual diagrams illustrating pilot signals in Patent Document 2. FIG. 21A is a diagram illustrating a portion of a frame format. FIG. 21B is a schematic diagram illustrating a relationship in frequency between spectrum of pilot signals and subcarriers. In FIG. 21A, the horizontal axis represents an arrangement of subcarriers in the frequency direction, and the vertical axis represents an arrangement of symbols in the time direction. Thus, each pilot carrier includes two adjacent subcarriers, and the pilot carriers are successively modulated with the same known data. In FIG. 21B, the horizontal axis represents frequencies, and the vertical axis represents spectral intensities. Spectrum fp1 and fp2 of pilot signals appear as emission line spectrum each of which is located at a middle between two adjacent subcarrier modulation spectrum.
Since the spectrum fp1 and fp2 of the pilot signals have an intermediate frequency component between subcarriers, the spectrum fp1 and fp2 are not likely to suffer from an interference from adjacent data carriers Dc1 to Dc4. Therefore, the ideal demodulation vector of the pilot signal is fixed to substantially a single point, and is known to a receiver. Therefore, in Patent Document 2, as is similar to the transmission channel characteristics estimation method of FIG. 18 in the first method of Patent Document 1, the amount of a change between the ideal demodulation vector of the pilot signal which is known to a receiver and a demodulation vector of an actually received pilot signal is calculated to estimate transmission channel characteristics.
It is also described that, in order to improve the precision of estimation of transmission channel characteristics, an adjacent subcarrier of a pilot carrier is assigned as a mask carrier and is not used for data transmission. By using the mask carrier, an interference from a data transmission signal which occurs in the pilot signal is suppressed, the ideal demodulation vector of the pilot signal is more correctly fixed to a single point, and the precision of estimation of transmission channel characteristics is improved.
Thus, each pilot carrier including two subcarriers is assigned, and the pilot carriers are successively modulated with the same known data, or further, by assigning an adjacent subcarrier of a pilot carrier as a mask carrier, the ideal demodulation vector of the pilot signal can be fixed to substantially a single point, so that transmission channel characteristics can be estimated.
However, in the first method of Patent Document 1, it is necessary to repeatedly modulate all subcarriers over 2K−1 symbols with a pilot signal which is known both to a transmitter and a receiver in order to arrange a continuous pilot signal. Since the ideal demodulation vector of a pilot signal can be fixed to a single point, the precision of estimation of transmission channel characteristics is high. However, since a transmission data signal cannot be inserted during transmission of a continuous pilot signal, frequency efficiency is deteriorated. In particular, when the overlapping symbol number K is large, the frequency efficiency is further reduced.
Also, in the second method of Patent Document 1, it is necessary to modulate all subcarriers over one or more and less than 2K−1 symbols with the same pilot signal which is known data in a receiver in order to arrange a short continuous pilot signal. Also, when a time required to modulate all subcarriers with the same pilot signal is short, the distance between the ideal demodulation vectors of pilot signals of two adjacent subcarriers at the same time may be small, so that the precision of estimation of transmission channel characteristics is deteriorated. Meanwhile, there is a large variation in the estimated value of transmission channel characteristics calculated for each symbol, and there is also a large error. Therefore, as compared to the first method of Patent Document 1, the frequency efficiency is improved, but the precision of estimation of transmission channel characteristics is reduced and there is a large variation in the estimated value and there is also a large error. Also, in order to prevent the deterioration of the precision of estimation of transmission channel characteristics due to the small distance between the ideal demodulation vectors of two adjacent subcarriers, it is necessary to increase the number of repetitions of a pilot symbol, the frequency efficiency is reduced as is similar to the first method of Patent Document 1.
Also, in the third method of Patent Document 1, it is necessary to modulate all subcarriers over two or more and less than 2K−1 symbols with the same pilot signal which is known data in a receiver in order to arrange a short continuous pilot signal. Also, when a time required to modulate all subcarriers with the same pilot signal is short, the distance between the ideal demodulation vectors of pilot signals of two successive symbols in the time direction in the same subcarrier may be small. Therefore, the precision of estimation of transmission channel characteristics is deteriorated.
Further, in Patent Document 2, it is necessary to use pilot carriers each of which includes two adjacent subcarriers. When a limited frequency resource is effectively used or when the total number of subcarriers is small, it is preferable that the number of pilot carriers, which cannot be used for data transmission, be small. Also, when there is even a small interference component from an OFDM/OQAM multicarrier modulation signal of a subcarrier adjacent to a pilot carrier, it is difficult to fix the demodulation vector of the pilot signal completely to a single point, and therefore, the precision of estimation of transmission channel characteristics is deteriorated. To prevent this, it is necessary to assign a subcarrier adjacent to a pilot carrier as a mask carrier. In this case, however, the number of subcarriers which cannot be used for data transmission increases, the frequency efficiency is further reduced.
Note that it is also contemplated that the transmission channel estimation method in the second method of Patent Document 1 and the frame format of Patent Document 2 may be combined. Specifically, regarding the frame format, pilot carriers each of which includes two adjacent subcarriers are used as in FIG. 21A, and the same known data is inserted into the pilot carriers successively. Regarding the transmission channel estimation method, as in FIG. 20, transmission channel characteristics are estimated based on demodulation vectors of pilot signals of two adjacent subcarriers.
However, even when such a combination technique is used, the problem with Patent Document 2 that at least two adjacent subcarriers are required to generate a pilot carrier, and the problem with the second method of Patent Document 1 that the precision of estimation of transmission channel characteristics is reduced due to the small distance between demodulation vectors of pilot signals of two adjacent subcarriers, cannot be solved.